Printing devices that create dots on a printing medium to print an image are widely used as the output device of various imaging devices. In these printing devices, an image is divided into tiny areas called pixels, and dots are created in these pixels. Such a printing device takes only either of two states, a dot-on state and a dot-off state, in each pixel, but makes areas of dense dot creation and areas of sparse dot creation in the whole image. For example, in the case of creation of blank ink dots on printing paper, areas of dense dot creation are dark areas and areas of sparse dot creation are bright areas. Adequate regulation of the density of dot creation according to the tone values of an object image gives a multi-tone printed image.
The following method is generally adopted in such a printing device to create dots at adequate densities according to the tone values of an object image. The procedure first carries out a preset series of image processing of an object image to be printed and converts image data into data representing the dot on-off state in each pixel (hereafter referred to as ‘dot data’). The adequate image processing of the object image generates dot data for creation of dots at the adequate densities according to the tone values of the image data. The procedure then supplies the dot data representing the dot on-off state to the printing device. The printing device creates dots in the pixels according to the received dot data. This method enables dots to be created at adequate densities according to the tone values of the image data and thereby prints a desired image.
This image printing method, however, takes a long time for transfer of processed data with an increase in number of pixels included in the object image. This prevents quick printing. With recent requirements for the enhanced picture quality and the increased image size, the number of pixels included in the image has been increasing. The high-speed printing of an image accordingly has difficulties. This problem is described, for example, in Japanese Patent Laid-Open Gazette No. 2000-15716.
Dots are created on a printing medium, such as printing paper, by dot formation elements mounted on a print head, for example, by nozzles for ejecting ink droplets. When a dot formation element has any deviation in dot creation, for example, a deviation of the hitting position of ink droplets for dot creation, raster lines formed by the dot creation element are different from raster lines formed by the other dot formation elements. This causes, for example, the occurrence of banding, which gives white streaks to a resulting print. Recent printing devices have thus formed each raster line by at least two of the multiple dot formation elements mounted on the print head. In a printing device that executes printing by forward and backward scans of the print head relative to the printing medium, each raster line is completed by multiple forward scans or backward scans. The procedure first uses one dot formation element to create dots at intervals (for example, alternately), shifts the relative position of the print head to the printing medium, and then uses another dot formation element to fill the intervals between the previously created dots and complete a raster line. This technique is called interlacing.
This technique of dot creation does not immediately use all the generated dot data but is required to store the dot data until completion of each raster line. The print head has multiple dot formation elements at intervals of several dots, and the printing device often adopts a technique of completing raster lines of a preset width by multiple forward scans or backward scans of the print head (interlacing technique). Combination of the interlacing technique with the interlacing technique drastically increases the quantity of dot data to be stored. A large memory capacity is then required for storage of the large quantity of dot data.
An image processing device (typically a computer) and a printing device may share a series of image processing. The printing device may alternatively carry out the whole series of image processing. In any case, it is required to expand dot data corresponding to the dot formation elements and store the expanded dot data. A large storage capacity is required for storing the dot data. In the case of combined use of the image processing device and the printing device for image printing, the image processing device may take charge of generation of dot data. The image processing device is typically actualized by execution of an application program for image processing on the computer. In the case of printing an image, a printer driver is activated to receive required data from the application program and generate data to be output to the printing device. When the computer has a sufficient memory capacity, the printer driver may expand and store a large quantity of dot data. This, however, undesirably increases the quantity of data transferred from the image processing device to the printing device and extends the time for data transfer.
The object of the invention is thus to eliminate the drawbacks of the prior art techniques discussed above and to provide a technique that reduces a required memory capacity for expansion of dot data and thereby carries out image processing with a high efficiency.